Traditional methods of cooling various components, such as electrical components, have often involved air convection cooling. In such cooling, heat dissipated by an electronic component is transferred to the surrounding air which carries the heat to an ultimate sink, generally, the room in which the component is located. The heat transfer process can be enhanced by increasing the surface area of contact between the component and the air, for example by using fins on the component or by mounting the component on a heat sink to which heat is transferred by conduction and which then dissipates heat by convection. The heat sink may itself have fins to promote convection. In addition, convection may occur by natural air circulation or by forcing the air to circulate by means of fans or blowers. Such traditional air convection cooling functions well, but has significant limitations.
For example, as modern day electronic equipment has become more sophisticated and more compact, electronic component density has steadily increased: this trend has been mirrored in integrated circuit packages in which functionality has been increased without a corresponding increase in package size by greatly increasing the number and density of active devices, such as transistors, within the packages.
As the number of active devices has increased within a fixed package volume, heat dissipated by these devices has also increased to the point where, in many modern day devices, the power density is so high that conventional air convection cooling methods, even using forced air circulation, cannot remove the dissipated heat sufficiently fast in order to keep the circuit package within its allowed operating temperature range. Further, forced air convection cooling methods also have practical limits because the high air flow required to provide sufficient cooling in many high-powered systems can create an unacceptable noise level in a relatively quite environment such as an office.
Consequently, several liquid cooling techniques have conventionally been used with high-power electronic components. These techniques generally fall into two broad groups comprising single phase and two phase cooling systems. In a single phase liquid cooling system, the liquid coolant remains as a liquid over the normal operating range of the system. This is in contrast to a two phase cooling system in which the liquid coolant changes from its liquid phase to a vapor phase during at least one point in the normal operating range.
An example of a single phase liquid cooling system which uses natural convection comprises a hermetic enclosure which encloses the heat-generating component device. The enclosure may be provided with external fins and is filled with a conductive liquid coolant. Heat is transferred from the heat dissipating component to the coolant mainly by conduction and from the coolant to the enclosure by natural convection. The enclosure itself may be cooled by circulating air around the outside of the enclosure. Such a cooling method is effective but involves additional problems, such as chemical incompatibilities between the component and the coolant and difficulty of maintaining the component.
Other single phase cooling systems do not immerse the heat-generating component directly in the liquid coolant, but instead confine the coolant in a container. Heat is conducted from the generating component through the container wall into the liquid coolant which then dissipates the heat by natural convection. An example of such a cooling system uses a sealed flexible bag which is filled with a liquid coolant. The bag may be constructed from a flexible plastic film which is relatively impermeable to both air and the liquid coolant. The bag is placed in contact with the heat-dissipating component and, since the bag is flexible, it conforms to the component shape and heat is transferred through the relatively thin plastic film to the coolant. Other embodiments of this type of system use metal inserts to more efficiently conduct the heat from heat-dissipating component to the coolant. Examples of coolant bag systems are shown in U.S. Pat. Nos. 4,997,032 and 5,000,256.
While such single phase systems can be useful in many situations, their heat transfer rate is still relatively low, and, consequently, they cannot be used with a high-power electronic components unless they have a relatively large volume which is incompatible with most compact electronic systems available today.
Single phase forced convection designs have also been used. In these latter designs, the heat-dissipating component is located inside a small enclosure through which liquid coolant is pumped. The coolant is generally recirculated between the component enclosure and a liquid-air heat exchanger (occasionally a liquid-liquid heat exchanger is used) where the heat is dissipated by either natural or forced convection. These latter cooling systems can dissipate large amounts of heat, but are subject to leaks and require pumps for operation.
Consequently, two phase liquid cooling systems have been used to overcome the problems of single phase systems. In a conventional two phase cooling system, a low boiling point liquid coolant is used; the liquid is vaporized or boiled by heat dissipated by the electronic component and the vapor travels to a condenser. In the condenser the coolant vapor is converted back into a liquid and the liquid is then returned to the heat dissipating component so that the boiling/condensing cycle can be repeated.
An example of a two phase device in which the heat dissipating component is directly immersed in the coolant is shown in U.S. Pat. No. 3,741,292. The heat dissipating component is located in a hermetic enclosure which contains a sufficient pool of low boiling point dielectric liquid coolant to partially fill the enclosure and immerse the heat dissipating component. The liquid is evaporated and the heat dissipated by the component and the resulting vapor is collected in the enclosure space located above the liquid pool. This enclosure space can have fins extending inwardly into the enclosure which fins serve as a condenser for the coolant vapor. The enclosure is also equipped with external fins which serve as an air cooled heat sink to cool the enclosure. As the vapor condenses it runs back into the liquid pool under the influence of gravity.
Another type of two phase cooling system which does not directly immerse the component in the coolant is a heat pipe system. A heat pipe consists of an elongated, hermetic container with thermally conductive walls, for example, a copper pipe is often used. One end of the container acts as an evaporator and the other end acts as a condenser. A wick or other capillary device extends along the length of the container - if a copper pipe is used, the wick often consists of a fine mesh screen extending along the inside of the pipe. The container is partially filled with a low boiling point liquid coolant and the residual non-condensing gases are purged. More particularly, during construction of the heat pipe, the air which normally fills the container is purged by boiling the coolant to drive off the air. The container is then sealed.
The evaporator end is mounted next to the heat dissipating component and heat is transferred by conduction through the container wall of the device. As the coolant evaporates or boils, the resulting vapor travels down the container to the condenser end where it condenses back to a liquid. The liquid is returned to the evaporator end by means of the wick (or alternatively by gravity).
The direct immersion enclosure and the heat pipe can transfer heat efficiently away from a heat dissipating component, but also have limitations. More specifically, both the direct immersion enclosure and indirect heat pipe two phase devices utilize rigid, hermetically sealed containers and, thus, the internal pressure of the devices does not remain equal to the ambient environmental pressure. For example, in a heat pipe device, most of the non-condensable residual gas is purged during manufacture to prevent excessive pressures in the device in the normal temperature operating range. Consequently, when the device is not operating, there usually exists a slight vacuum in the device at normal ambient temperature. Accordingly, the device is prone to leaks, and, if a leak occurs, air will be drawn into the device. Later, when the device is operating, the increased pressure produced by the air may drive some of the liquid out of the container. Consequently, the devices are not reliable in an environment where long term maintenance is impossible.
Further, due to the fact that the container walls are rigid, as the liquid coolant vaporizes, the pressure inside the prior art devices increases, in turn, increasing the coolant boiling point. The increase in boiling point is exacerbated by the presence of residual air and air introduced into the system by leaks. Consequently, the coolant liquid within the devices does not have a single boiling point but rather a range of boiling points and the devices do not operate at a single temperature but instead operate over a relatively broad range of temperatures.
Further, such devices are often subject to a phenomenon called "overshoot". Overshoot occurs during device warmup because the liquid coolant does not begin to boil when it reaches its boiling point. Instead, the liquid temperature continues to increase until the temperature is significantly over the normal coolant boiling temperature and boiling suddenly erupts. When boiling finally does occur, the device temperature returns to its normal operating temperature range. Overshoot is highly undesirable as it stresses the components to be cooled and, in some cases, may cause a component to temporarily operate outside its normal operating temperature range.
In addition, two-phase cooling devices which rely solely on gravity to return condensate to an evaporation area suffer from the additional drawback that they can only operate in a limited range of orientations. That is, if the vapor condenses in a region that is "downhill" from the evaporation area, condensate will not return to the evaporation region, phase-change cooling will not take place, and catastrophic device failure may result. Although a wick may be employed to return condensate to the evaporation area, a wick will provide only limited transport capacity. It is also possible to use a pump to provide sufficient condensate-delivery capacity, but only at the price of additional expense and complexity.
Accordingly, it is an object of the present invention to provide a cooling system for electronic components which has a high heat transfer rate and can maintain the electronic component within a relatively narrow temperature range. Additional objects achieved by various embodiments of the invention are to provide cooling systems which do not require maintenance, are reliable over a long time period and which are not subject to overshoot. A further object is to provide a cooling system which can operate even if it is turned "upside-down".